Electrolysis involves an electrochemical oxidation-reduction reaction associated with the decomposition of a compound. In an electrolysis cell an electrical current passes between two electrodes and through an electrolyte, which can be the compound alone, e.g., sodium chloride or the compound dissolved in a liquid solvent, e.g., aluminum dissolved in cryolite, such that a metallic constituent of the compound is reduced together with a corresponding oxidation reaction. The current is passed between the two electrodes from an anode to a cathode to provide electrons at a requisite electromotive force to reduce the metallic constituent which usually is the desired electrolytic product, such as in the electrolytic smelting of metals. The electrical energy expended to produce the desired reaction depends upon the nature of the compound and the composition of the electrolyte. However, in practical application, the cell power efficiency of a particular electrolytic or reduction cell design can result in wasted energy depending on factors such as, inter alia, cell voltage and current efficiency.
Generally speaking, aluminum is produced by electrolysis of aluminum compounds such as aluminum oxides or salts or other compounds in a molten salt bath. Typically, aluminum is produced by the Hall-Heroult electrolytic production process wherein aluminum oxide dissolved in molten cryolite is electrolyzed at a temperature of from 900.degree. C. to 1000.degree. C. During the reduction process molten aluminum is electrolyzed out of the aluminum oxide-cryolite melt and is periodically or continually withdrawn from the reduction cell.
A commonly utilized reduction cell for the manufacture of aluminum is the classic Hall-Heroult design, which utilizes carbon electrodes and a substantially flat carbon-lined bottom which functions as part of the cathode system. An electrolyte typically used in the production of aluminum by electrolytic reduction of alumina consists primarily of molten cryolite with dissolved alumina which may contain other materials such as fluorspar, aluminum fluoride, and other metal fluoride salts.
Molten aluminum resulting from the reduction of alumina is most frequently permitted to accumulate in the bottom of the receptacle forming the reduction cell, as a molten melt pad or pool over the carbon-lined bottom thus forming a liquid metal cathode. Carbon anodes extending into the receptacle and contacting the molten electrolyte are adjusted relative to the liquid metal cathode. Current collector bars such as steel are generally imbedded in the carbon line cell bottom and complete the connection through the cathodic system.
While the design and sizes of Hall-Heroult reduction cells vary, all have relatively low energy efficiencies, ranging from about 35 to about 45%, depending upon cell geometry and mode of operation. Thus, while the theoretical power requirement to produce one pound of aluminum is 2.85 kw hours, in practice power usages range from 6 to 8.5 kw hours per pound of aluminum with an industry average of about 7.5 kw hours per pound of aluminum.
Much of the voltage drop through a reduction cell occurs in the electrolyte and is attributable to electrical resistance of the electrolyte, or electrolytic bath, across the anode-cathode distance. The bath electrical resistance or voltage drop in conventional Hall-Heroult cells for the electrolytic reduction of aluminum from alumina dissolved in a molten cryolite bath includes a decomposition potential, i.e. energy in aluminum production and an additional voltage attributable to heat energy generated in the inter-electrode spacing by the bath resistance, which heat energy generally is discarded. Such discarded heat energy typically makes up 35 to 40% of the total voltage drop across the cell, and in comparative measure, as much as up to twice the voltage drop attributable to decomposition potential.
To minimize voltage drop and optimize cell efficiency the gap between the anode and surface of the aluminum pad should be maintained as small as possible, preferably not more than about 3 cm. This desirable close anode-cathode spacing is difficult to maintain during the magnetic induction currents which cause large perturbations in the molten aluminum pad which increases the risk of short circuiting the system by contact between the molten aluminum and the anode. For example, in a typical cell the spacing between the anode and surface of the molten aluminum cannot, as a practical manner, be maintained at less than about 4 cm.
Reducing the anode-cathode separation distance is one way to decrease energy loss. However, whenever the anode-cathode distance is reduced, short circuiting of the anode and cathode must be prevented. In minimizing the space between the anode and cathode, displacement of the metal in the aluminum pad caused by magnetic forces associated with the electrical currents employed in electrolysis must be carefully considered. Thus, to prevent shortening the anode-cathode separation must always be slightly greater than the peak height of the displaced molten product in the cell.
Another adverse effect which results from reducing the anode-cathode distance is a significant reduction in current efficiency of the cell when the metal produced by the electrolysis at the cathode is oxidized by contact with the anode product. For example, in the electrolysis of aluminum from alumina dissolved in cryolite, aluminum metal produced at the cathode can be oxidized readily back to alumina or aluminum salt by close proximity to the anodically produced carbon oxide. A reduction in the anode cathode separation distance provides more contact between the anode product and the cathode product and significantly accelerates the reoxidation of reduced metal, thereby decreasing current efficiency.
A consumable anode, such as the carbon anode conventionally used in the production of aluminum in a conventional reduction cell presents a substantial obstacle in achieving a precise control of the interelectrode spacing. In the conventional reduction cell oxygen gas produced at the anode combines with the carbon of the anode itself to form carbon oxide, such as carbon monoxide and carbon dioxide gas. Oxidation of the anodes together with air burning of the anodes consumes about 0.45 pounds of carbon for each pound of aluminum produced. This carbon loss necessitates careful monitoring of the anode height and frequent adjustment in conventional reduction cell practice.
Refractory hard metals (RHM) were first utilized for cathode constructions in aluminum reduction cells in the early 1950's. RHM materials in pure form are very resistant to molten alumina in cryolite found in an aluminum reduction cell and, moreover, generally have higher electrical conductivities than the conventional carbon products used in reduction cells. Additionally, RHM materials, and particularly TiB.sub.2, are readily wet by molten aluminum, whereas carbon products normally used are not.
Although the early use of RHM materials in aluminum reduction cells was conceptionally a significant improvement, such use was fraught with practical problems and, as a result, the development of RHM cathodes has only recently led to any significant use of these materials in reduction cells.
The present invention is an improvement over previous RHM cathode elements and associated reduction cells which allows for operation of commercial cells at significantly reduced anode-cathode separation distances without reducing the cell current efficiency, while at the same time significantly reducing the specific energy consumption.